There are many fields in which it is desirable to use polyurethane coating compositions for purposes of protection and for aesthetic reasons.
For reasons of economy, it is important that such coating compositions be applied rapidly and efficiently. In particular, with the constant striving for higher productivity in industry, methods of applying coatings (e.g., paints) to yield a standard film thickness in two coats instead of three or more and which still produce a serviceable coating is clearly a desirable goal to those concerned with application of coatings in production.
In the automotive industry, for example, which is one of the important applications for polyurethane coatings, the problem of coating application acquires specific urgency. Competitive pressure requires the original production finishing of automobiles to make the most effective use of labor and materials. Economizing production line space and the capability of increasing speed of completing unit operations such as painting therefore become important. Topcoat finishing, for example, that can achieve the same protective film thickness and beauty with two spray applications when three or more are standard practice, is regarded as a valuable modification.
Prior to this invention, commercial polyurethane enamel topcoats required at least three spray applications, particularly when pigmentation contained metallic flake, to deposit films of requisite thickness and appearance. This invention concerns the solution of the above-mentioned problems and additionally realizes further advantages to be mentioned below.